1. Field of the Invention
This invention relates to magnetic thin film heads (TFH) for recording and reading magnetic transitions on a moving magnetic medium. In particular, the invention relates to a method of selectively etching the seed-layer, or metallization-layer, between the coil turns, or winding, without attacking the coil turns, the pole gap, or the insulation.
2. Background of the Prior Art
Magnetic TFH transducers are known in the prior art. See, e.g. U.S. Pat. Nos. 4,016,601; 4,190,872; 4,652,954; 4,791,719. In operation of a typical device utilizing a TFH transducer, a moving magnetic storage medium is placed near the exposed pole tips of the TFH transducer. During the read operation, the changing magnetic flux of the moving storage medium induces changing magnetic flux upon the pole tips and gap between them. The magnetic flux is carried through the pole tips and yoke core around spiralling conductor coil winding turns located between the yoke arms. The changing magnetic flux induces an electrical voltage across the conductor coil. The larger the number of coil turns, the larger this voltage. The electrical voltage is representative of the magnetic pattern stored on the moving magnetic storage medium. During the write operation, an electrical current is caused to flow through the conductor coil. The current in the coil induces a magnetic field across the gap between the pole tips. A fringe field extends into the nearby moving magnetic storage medium, inducing (or writing) a magnetic domain (in the medium) in the same direction. Impressing current pulses of alternating polarity across the coil causes the writing of magnetic domains of alternating polarity in the storage medium.
In the manufacturing of TFH transducers for magnetic recording, a large number of devices are usually fabricated simultaneously in deposited layers on a ceramic wafer. When completed, the wafer is cut (or diced) and machined into the individual transducers. The four main elements of a TFH transducer, roughly in the order in which they are deposited, are the bottom magnetic pole, the flux gap material to provide spacing between the bottom and top magnetic pole tips, one or more levels of electrical conducting coil winding interposed within insulation layers, and the top magnetic pole. One or more levels of coil winding may be constructed.
Usually the magnetic poles are made of a nickel-iron alloy (called Permalloy), and the coil winding is made of copper. To form each level of the coil winding, a continuous metallic electrical conductive layer called "metallization-layer" or "seed-layer", is first deposited over the entire wafer's surface (consisting at this stage of gap and insulation layers). The seed-layer consists of a single-layer or multiple-layer and is commonly deposited by vacuum deposition techniques, such as sputtering or evaporation. The common seed-layer for coil deposition consists of Cu/Cr or Cu/Ti. Cr or Ti are used as the bottom layer to enhance adhesion to the insulation and alumina surfaces. The seed-layer renders the wafer's surface electrically conducting and thus appropriate for successive electroplating. Following the deposition of the seed-layer, a photoresist is applied to the surface and is photolithographically patterned to expose the seed-layer where the spiral coil winding turns are to be formed. The wafer is then placed in an electroplating cell and the required metal (Cu) is electroplated through the photoresist mask onto the exposed areas of the seed-layer. The rest of the area on the wafer is protected by the non-conductive photoresist, and there is no electrodeposition in this area. After electrodeposition, the photoresist is stripped-off, leaving plated area connected by seed-layer. The seed-layer must be removed from the area previously protected by the photoresist (and which was not electroplated), so that it will not short between areas of the electroplated metal. The common ways to remove the seed-layer are by sputter-etching or ion-milling (both of which are vaccum techniques), or by wet chemical etching (with two different etchants in succession: first for Cu, then for Cr or Ti). The formation of a coil winding is generally discussed in U.S. Pat. Nos. 4,539,616 4,127,884; 4,165,525; and 4,219,853.
It is here that problems have in the past existed. The coil winding wall is tall relative to the spacing between the coil turns, so that the seed-layer is located in relatively deep, narrow valleys between the individual turns. Sputter-etching of the entire area is the usual process to remove the inter-turn seed-layer, but it requires, to assure that all of the seed-layer has been removed, also the removal of a substantial thickness of the electrodeposited copper winding and other features of the partially completed head. The same problem exists if ion-milling, conventional wet chemical etching, or any other indiscriminate or non-selective process is used over the entire surface.
The vacuum techniques (sputter-etching and ion-milling) are costly and time consuming. They require expensive equipment, and the throughput is low. Also, these techniques are indiscriminate or non-selective. That is to say, they result in removal of material from the entire exposed surface of the wafer. The seed-layer, as well as a layer of similar thickness in the plated areas, is etched or milled off. To ensure complete removal of the seed-layer, the etching or milling is usually carried out for a longer time than is necessary. This extra time is used to ensure consistently complete removal of the seed-layer from manufacturing run to manufacturing run. However, the extra etching or milling time results in partial removal of previously deposited layers, such as the gap, insulation, and plated layers.
The undesireable etching of the gap and insulation has been described in U.S. Pat. No. 4,652,954, and is referred to as the "gap-wedge" and "zero-throat recession", respectively. U.S. Pat. No. 4,652,954 teaches the use of a gap protective layer, such as Cr, to protect the gap during the etching. Sputter-etching or ion-milling of the insulation layer (commonly consisting of cured photoresist) results in hard to control and non-reproducible "zero-throat" height. However, the gap protective layer of U.S. Pat. No. 4,652,954 does not prevent insulation recession due to sputter-etching or ion-milling. The plated metal coil (Cu) is also affected by the non-selective etching, resulting in hard to control and nonreproducible thickness of the Cu in the coil windings and, correspondingly, variable coil resistance.
Because the vacuum etching techniques are dependent on bombardment by accelerated ions, they are highly directional and may also cause the seed-layer to impregnate the layer below. In areas where the seed-layer is shadowed by elevated features with wall angle larger than 90.degree. (such as the electroplated features), the seed-layer may not be completely removed. The incomplete removal of the seed-layer and/or the degradation of insulation may cause leakage currents or elecrical shorts or electrical discharge from head to disk during contact, as well as presenting potential reliability problems. The latter, for instance, might be caused by galvanic dissimilar metals in exposed pole-tips corners, which significantly increase the susceptibility of pole-tips to environmental corrosion. A common example is the presence of Cu seed-layer residues along the edges of poles. Such residues, sometimes called "wings", are most undesireable. If, on the other hand, an attempt is made to remove all traces of the seed layer by prolonged vacuum etching, there is a danger of removing too much coil material, because it is subject to the same bombardment and etches more rapidly.
The conventional non-selective wet chemical etching combines two steps of different etchants, first for CU, then for Cr (or Ti). During the wet etching of the CU seed-layer, the plated Cu coil windings are also etched, thereby decreasing their width and thickness and increasing the coil resistance. The time to clear (or etch) the Cu seed-layer from between the coil winding turns varies considerably from wafer to wafer, from lot to lot, and across the wafer itself. It thus results in poor or no control of the coil winding width and thickness and, correspondingly, the coil resistance. Also, additional wet etchant is required for the Cr or Ti adhesion layer.